DESCRIPTION(provided by the applicant): Diminution of thymic tissue and function is thought to be crucial in the compromised immunity seen in the aged, but the mechanisms of this thymic involution are not yet understood. Our preliminary data analyzing the expression of gamma-phosphorylated H2AX protein (gH2AX), a marker for DNA breaks, in mouse thymocyte subpopulations using flow cytometric quantitation show that the frequency of thymocytes expressing gH2AX is substantially increased in aged thymocytes at all stages of development. We hypothesize that this increased frequency of DNA damage in aged thymocytes may be caused by inability to repair DNA breaks such as those caused during T cell receptor (TCR) locus recombination, and/or by an intrathymic increase in apoptotic signals. Both possible explanations have important implications for the process of thymic involution in the aged. We will ascertain, in aim 1, the number of gH2AX foci per cell in young versus aged thymocytes using confocal fluorescence microscopy to determine whether non-repair of TCR breaks or induction of apoptotic breaks is dominant. We will also test, using fluorescence in situ hybridization (FISH), if the TCR loci are sites of gH2AX localization in thymocytes from aged mice. We will determine, in aim 2, if DNA repair complexes containing gH2AX in the aged are qualitatively different from those found in young thymocytes by examining the DNA repair molecules recruited to the gH2AX foci. Together, these data will elucidate the role of DNA damage and repair mechanisms in the thymic involution of the aged.